Heating Energy Demands and Sustainable Generation Concepts for Agricultural Greenhouses

Transcription

1 University of Windsor Scholarship at UWindsor Electronic Theses and Dissertations 2017 Heating Energy Demands and Sustainable Generation Concepts for Agricultural Greenhouses Lucas Macrae Semple University of Windsor Follow this and additional works at: Recommended Citation Semple, Lucas Macrae, "Heating Energy Demands and Sustainable Generation Concepts for Agricultural Greenhouses" (2017). Electronic Theses and Dissertations This online database contains the full-text of PhD dissertations and Masters theses of University of Windsor students from 1954 forward. These documents are made available for personal study and research purposes only, in accordance with the Canadian Copyright Act and the Creative Commons license CC BY-NC-ND (Attribution, Non-Commercial, No Derivative Works). Under this license, works must always be attributed to the copyright holder (original author), cannot be used for any commercial purposes, and may not be altered. Any other use would require the permission of the copyright holder. Students may inquire about withdrawing their dissertation and/or thesis from this database. For additional inquiries, please contact the repository administrator via or by telephone at ext

2 Heating Energy Demands and Sustainable Generation Concepts for Agricultural Greenhouses By Lucas M. Semple A Thesis Submitted to the Faculty of Graduate Studies through the Department of Civil and Environmental Engineering in Partial Fulfillment of the Requirements for the Degree of Master of Applied Science at the University of Windsor Windsor, Ontario, Canada Lucas M. Semple

3 Heating Energy Demands and Sustainable Generation Concepts for Agricultural Greenhouses by Lucas M. Semple APPROVED BY: Dr. Graham Reader Department of Mechanical, Automotive & Materials Engineering Dr. Paul Henshaw Department of Civil and Environmental Engineering Dr. Rupp Carriveau, Advisor Department of Civil and Environmental Engineering Dr. David S-K. Ting, Co-Advisor Department of Mechanical, Automotive & Materials Engineering April 11, 2017

4 DECLARATION OF PREVIOUS PUBLICATIONS This thesis includes three original papers that have been previously published/ submitted for publication in peer reviewed journals, as follows: Thesis Chapter Publication Title Publication status Chapter II L. M. Semple, R. Carriveau & D. S-K. Ting (2017). Assessing Heating and Cooling Demands of Closed Greenhouse Systems in a Cold Climate, International Journal of Energy Research Chapter III L. M. Semple, R. Carriveau & D. S-K. Ting (2016). Potential for Large-Scale Solar Collector System to Offset Carbon-Based Heating in the Ontario Greenhouse Sector, International Journal of Sustainable Energy, DOI: / Accepted for Publication Published Chapter IV L. M. Semple, R. Carriveau & D. S-K. Ting (2017). A Techno-Economic Analysis of Seasonal Thermal Energy Storage for Greenhouse Applications, Energy and Buildings Under Review I certify that I have obtained a written permission from the copyright owner(s) to include the above published material(s) in my thesis. I certify that the above material describes work completed during my registration as a graduate student at the University of Windsor. Within the above material I carried out all background research, methodology, topic selection, simulation design and analysis, interpretation of results, and preparation of text, with guidance and review provided by Dr. Rupp Carriveau and Dr. David S-K. Ting. I declare that, to the best of my knowledge, my thesis does not infringe upon anyone s copyright nor violate any proprietary rights and that any ideas, techniques, quotations, or any other material from the work of other people included in my thesis, published or otherwise, are fully acknowledged in accordance with the standard referencing practices. Furthermore, to the extent that I have included copyrighted material that surpasses the bounds of fair dealing within the meaning of the Canada Copyright Act, I certify that I have obtained a written permission from the copyright owner(s) to include such material(s) in my thesis. iii

5 I declare that this is a true copy of my thesis, including any final revisions, as approved by my thesis committee and the Graduate Studies office, and that this thesis has not been submitted for a higher degree to any other University or Institution. iv

6 ABSTRACT There is currently a global effort to reduce dependency on carbon-based fuels and move towards more sustainable practices utilizing renewable energy sources. This is in part due to the detrimental effects to the environment and climate change caused by the procurement and combustion of these fuels. Buildings account for a significant portion of global final energy use for heating and cooling purposes. This work focuses on the agricultural greenhouse sector in a cold climate, where significant heating demands are present and are typically met by utilizing significant amounts of natural gas. The heating demand of these structures is examined as well as sustainable generation concepts that have the potential to reduce this dependency on carbonbased fuels. Chapter II investigates the potential of closed greenhouse systems in a cold climate, where active cooling is implemented and the heat removed is stored for later use. It is determined that the annual cooling demand is equal to or greater than the heating demand in each of the cold climates examined and the use of a high-insulating cover material would be most suitable due to the significant reduction in annual heating demand. Chapter III analyses the ability of a largescale solar collector system to cover a significant portion of the greenhouse heating demand during the summer months. It is determined that a solar collector system with total area of ~575 m 2 is able to cover 97% of the heating demand during the month of July and approximately 27% of the annual demand of a 0.4 hectare greenhouse. By replacing natural gas CO 2 equivalent emissions are reduced by about 95 tonnes/ year and a payback period of about 10 years is achievable with carbon tax at a rate of $200/ tonne of CO 2 equivalent emissions. Finally, Chapter IV simulates the performance of a large-scale solar collector system with seasonal thermal energy storage (STES), where year-round heat is supplied by the system. High and lowtemperature systems are able to cover approximately 64% of the annual heating demand and achieve a system coefficient of performance of about 21.7 and 2.9, respectively. The systems are able to reduce CO 2 equivalent emissions by ~220 tonnes / year and a payback period of about 7 years is achievable with a 70% subsidy and carbon tax at a rate $200/ tonne of CO 2 equivalent emissions. v

7 DEDICATION I dedicate this work to my beautiful wife who has supported me throughout this endeavour and to my family for their love and confidence. vi

8 ACKNOWLEDGEMENTS I would like to thank my advisors Dr. Rupp Carriveau and Dr. David S-K. Ting for their guidance and support during the preparation of this work. They have been a pleasure to work with and learn from and I hope our relationship continues to grow in the future. vii

9 TABLE OF CONTENTS DECLARATION OF PREVIOUS PUBLICATIONS... iii ABSTRACT...v DEDICATION... vi ACKNOWLEDGEMENTS... vii LIST OF TABLES... xi LIST OF FIGURES... xii CHAPTER I...1 Introduction Background Methodology...2 References...2 CHAPTER II...3 Assessing Heating and Cooling Demands of Closed Greenhouse Systems in a Cold Climate Introduction Greenhouse Model Energy and Mass Balance Model Controls Reference Greenhouse Results and Discussion Reference Greenhouse Closed Greenhouse Closed Greenhouse in Different Canadian Settings...13 viii

10 4.4 Discussion Sensitivity Analysis Conclusion...18 References...19 CHAPTER III...21 Potential for Large-Scale Solar Collector System to Offset Carbon-Based Heating in Ontario Greenhouse Sector Background Introduction Greenhouse Model Interior Microclimate Energy and Mass Balance Heating Energy Demand Grower Data Solar Collector System Sensitivity Analysis Economic Analysis Current Economics Carbon Emission Reductions and Carbon Tax Discussion Conclusion...39 References...39 CHAPTER IV...42 A Techno-Economic Analysis of Seasonal Thermal Energy Storage for Greenhouse Applications Introduction Methodology Greenhouse Load Profile...44 ix

11 2.2 System Components General Solar Collector System Borehole Thermal Energy Storage System Controls High-Temperature System Controls Low-Temperature Pump Power Results System Operation BTES Performance Levelized Cost of Electricity Sensitivity Analysis Discussion Conclusion...64 References...64 CHAPTER V...67 Conclusions and Recommendations Summary and Conclusions Recommendations...68 APPENDIX A...70 Permission for Previously Published Works...70 APPENDIX B...71 Component Variables Utilized in TRNSYS Simulations Vita Auctoris...74 x

12 LIST OF TABLES Chapter II: Assessing Heating and Cooling Demands of Closed Greenhouse Systems in a Cold Climate Table 1 - Annual Heating and Cooling Demands for Closed Greenhouse with Differing Locations and Cover Materials Chapter III: Potential for Large-Scale Solar Collector System to Offset Carbon-Based Heating in Ontario Greenhouse Sector Table 1 - Daily Average Radiation for Windsor, Ontario Chapter IV: A Techno-Economic Analysis of Seasonal Thermal Energy Storage for Greenhouse Applications Table 1 - System Performance Results Table 2 - Estimated System Costs APPENDIX B: Component Variables Utilized in TRNSYS Simulation Table B-1 - Chapter III Potential for Large-Scale Solar Collector System to Offset Carbon-Based Heating in Ontario Greenhouse Sector - TRNSYS Component Variables Table B-2 - Chapter IV A Techno-Economic Analysis of Seasonal Thermal Energy Storage for Greenhouse Applications - TRNSYS Component Variables xi

13 LIST OF FIGURES Chapter II: Assessing Heating and Cooling Demands of Closed Greenhouse Systems in a Cold Climate Figure 1 - General Schematic Diagram of Energy Flow in Open and Closed Greenhouse...5 Figure 2 - Components of TRNSYS Simulation...6 Figure 3 - Crop (Lower) Zone Coupling Air Flow...9 Figure 4 - Actual and Modelled Energy Usage for the Reference Greenhouse Figure 5 - Monthly Heating Demand and Ventilation Cooling for Simulated Open Reference Greenhouse Figure 6 - Monthly Heating and Cooling Demand for Closed Reference Greenhouse Figure 7 - Annual Heating and Cooling Demand for Closed Greenhouse with Differing Cover Materials. 13 Figure 8 - Temperature Data Figure 9 - Closed Greenhouse Annual Heating and Cooling Demands for Differing Locations and Cover Materials Figure 10 - Sensitivity Analysis Heating Demand for Month of January Figure 11 - Overall Heating System Efficiency Chapter III: Potential for Large-Scale Solar Collector System to Offset Carbon-Based Heating in Ontario Greenhouse Sector Figure 1-3-Dimensional Greenhouse Model Figure 2 - Schematic Layout of Greenhouse Microclimate Simulation Figure 3 - Crop (Lower) Zone Coupling Air Flow Figure 4 - Temperature Data for Windsor, Ontario Figure 5 - Modelled Greenhouse Heating Energy Demand xii

14 Figure 6 - Energy Usage Data Obtained from Regional Growers Figure 7 - Monthly Heating Energy Demand Figure 8 - Schematic Layout of Solar Collector System Figure 9 - Horizontal Solar Radiation for Windsor, Ontario Figure 10 - Solar Collector System Operating Data Figure 11 - Heating Supplied by Solar Collector System (575 m 2 ) and Required Auxiliary Heating Figure 12 - Heating Supplied by Collector System with Decreasing Collector Slope Figure 13 - Sensitivity Analysis Annual Heating Supplied by Solar Collector System...37 Chapter IV: A Techno-Economic Analysis of Seasonal Thermal Energy Storage for Greenhouse Applications Figure 1 - General View of Proposed System...43 Figure 2 - Annual Temperature Data for Windsor, Ontario Figure 3 - Annual Solar Radiation for Windsor, Ontario Figure 4 - Heating Demand Profile from Model Figure 5 - Monthly Heating Energy Demand Figure 6 - General Borehole Layout Figure 7 - Schematic Layout of Investigated Systems Figure 8 - Average Ground Temperature for High and Low-Temperature Systems Figure 9 - Collector and Buffer Storage Tank Temperatures Beginning June 20 th at midnight Figure 10 - Collector and Buffer Storage Tank Temperatures Beginning November 28 th at midnight Figure 11 - Heating Supplied and Required Auxiliary Heat Figure 12 - Monthly Energy Transfer between System Components Figure 13 - System Performance with Varying Number of Boreholes xiii

15 Figure 14 - BTES Efficiency, High-Temperature System Figure 15 - BTES Efficiency, Low-Temperature System Figure 16 - Percentage of Heating Covered and Levilized Cost of Electricity (LCOE) with Varying Collector Areas Figure 17 - Sensitivity Analysis Annual Heating Supplied by System Figure 18 - Simple Payback Period with varying Carbon Tax (Natural $5/GJ) Figure 19 - Simple Payback Period with varying Natural Gas Cost (Carbon $60/Tonne CO 2e ) Figure 20 - Simple Payback Period with varying Capital Cost Subsidy xiv

16 CHAPTER I Introduction 1.0 Background The need for progress towards a low-emission economy is increasingly better understood. To avoid dangerous climate change, greenhouses gas emissions need to stabilize in 5~10 years and approach zero by the second half of the century [1]. Heat supplied to buildings, and especially those in cold climates, is identified as a key user of final energy. Heating energy demands throughout the industrialized world are generally met by combusting a carbonbased fuel such as natural gas, coal, oil, or propane. Combustion of these fuels results in the release to the atmosphere of what are referred to as greenhouse gases; carbon dioxide, methane and nitrous oxide [2]. There are also emissions associated with the procurement of these finite resources and processing into usable forms. Furthermore, these resources are not readily available throughout the world and further emissions are produced during transportation to the end-user. This presents a pressing need to accelerate the development and deployment of advanced clean energy technologies in order to address the global challenges of energy security, climate change and sustainable development [3]. The topic of this work is the agricultural greenhouse sector, with a focus on greenhouses in cold climates. A greenhouse is an enclosed structure, covered with glass or a transparent plastic, which creates a favourable microclimate for crop growth. As the cover materials are designed for maximum light transmission, the insulating properties of a greenhouse structure are far inferior to those of a conventional building. Whether seasonal or yearround harvesting schedules are implemented by a grower, there is a portion of the winter season where crops are present in the greenhouse. As individual operations can easily exceed 20 hectares in plan area and a difference between indoor and outdoor temperatures of up to 40 C can be experienced, greenhouses in cold climates have significant heating demands. Heating systems typically represent the highest consumption of energy in greenhouses and can account for up to 90% of the total demand [4,5,6]. There are several design and operational strategies for energy conservation available to greenhouse operators and these have been well studied and reviewed [7,8]. Utilizing thermal curtains, a thermal mass on the interior of the greenhouse, adjusting set-point temperatures, and proper placement of heating pipes are examples of conservation techniques. This work aims to look beyond conservation techniques and examine greenhouse systems of the future. The closed greenhouse concept described in Chapter II has been implemented to a limited extent in Europe but offers the possibility to significantly reduce the heating energy demand. With widespread resources throughout the world, solar energy is a low-emission alternative to conventional carbon-based fuels for space heating needs. Chapters III and IV assess the potential of large-scale solar collector systems with and without seasonal thermal energy storage (STES) to cover the summer and year-round heating load. 1

17 2.0 Methodology The work presented herein is in large part based on the operational characteristics of greenhouses in Southwestern Ontario; a region with the highest density of greenhouses in North America. Information has been obtained from published research on greenhouses from international sources and has been supplemented with more detailed information from meetings with regional growers. Energy usage data for heating purposes has been obtained from regional growers in Southwestern Ontario. The software program TRNSYS, a Transient System Simulation Program, was utilized to simulate the greenhouse microclimate and heating systems. TRNSYS is a complete and extensible simulation environment for the transient simulation of thermal and electrical systems, including multi-zone buildings [9]. Simulation of the greenhouse interior microclimate has been used in conjunction with the obtained energy usage data to determine the heating load profile for a greenhouse. The load profile has then been utilized to assess the performance of solar collector systems with and without seasonal thermal energy storage. Sensitivity analyses have been carried out in each respective section to assess the stability of the model in relation to key input parameters. References [1] United Nations Environment Program (UNEP) (2014). The Emissions Gap Report 2014: A UNEP Synthesis Report [2] United States Environmental Protection Agency (USEPA) (2013). Inventory of U.S. Greenhouse Gas Emissions and Sinks: [3] International Energy Agency (2012). Technology Roadmap Solar Heating and Cooling, Paris, France [4] Fernandez, M.D., Rodrıguez, M.R., Maseda, F., Velo, R., Gonzalez, M.A. (2005). Modelling the Transient Thermal Behaviour of Sand Substrate heated by Electric Cables, Biosystems Engineering 90 (2), [5] Hemming, S., Kempkes, F.L.K., Janse, J. (2012). New Greenhouse Concept with High Insulating Double Glass and New Climate Control Strategies Modelling and First Results from a Cucumber Experiment, Acta. Hort. 952, [6] Sturm, B., Maier, M., Royapoor, M., Joyce, S. (2014). Dependency of production planning on availability of thermal energy in commercial greenhouses-a case study in Germany, Applied Thermal Engineering 71, [7] Sethi, V.P., Sumathy, K., Lee, C., Pal, D.S. (2013). Thermal modeling aspects of solar greenhouse microclimate control: A review on heating technologies, Solar Energy 96, [8] Cuce, E., Harjunowibowo, D., Cuce, P.M. (2016). Renewable and sustainable energy saving strategies for greenhouse systems: A comprehensive review, Renewable and Sustainable Energy Reviews 64, [9] Solar Energy Laboratory (SEL), University of Wisconsin-Madison (2014). TRNSYS 17 Manual, Volume 1, pp.8. SEL, Madison. 2

18 CHAPTER II Assessing Heating and Cooling Demands of Closed Greenhouse Systems in a Cold Climate Lucas Semple +, Rupp Carriveau +, David S-K. Ting ± + Department of Civil and Environmental Engineering, University of Windsor, Windsor, Ontario, Canada ± Department of Mechanical Automotive and Materials Engineering, University of Windsor, Windsor, Ontario, Canada 1.0 Introduction A greenhouse is an enclosed structure that creates a favourable micro-climate for crop production. They can produce much higher crop yields with more consistent crop quality than field crops [1]. Energy costs are a major economic factor in greenhouse operations. Heating systems typically represent the highest consumption of energy in greenhouses and can account for up to 90% of the total demand [2,3,4]. Heating is conventionally accomplished by combusting a carbon-based fuel in a boiler and a hydronic heating system is utilized to distribute heat within the greenhouse. During the summer months natural or forced ventilation strategies are implemented to avoid overheating and dehumidify the indoor air. A general schematic of the open greenhouse energy flow is shown in Figure 1A. Indoor air temperature has long been recognized as the most significant factor influencing plant development, while net production is mostly influenced by available solar radiation [5]. Humidity control is another important thermal condition affecting the growth of crops [6]. In an effort to reduce heating demand and fossil-fuel usage, and in turn related emissions from greenhouse operations, optimal design and operation of greenhouses has been thoroughly studied by numerous researchers. Greenhouse cover material properties directly affect short-wave solar radiation transmission to the greenhouse interior and long-wave thermal radiation losses to the sky. Zhang et al [7] compared double and single polyethylene (PE) covering materials to single glass and found double-pe covering materials could result in significant energy savings over single glass. Furthermore, high insulating double-glass covering materials have been observed to reduce annual energy consumption by 25-33% [8]. However, due to the importance of solar radiation on crop production, increased insulating properties that negatively affect transparency have to yield large savings to be economically viable [9]. In addition, improved cover insulating properties lead to higher greenhouse humidity levels and the anticipated decrease in energy use for heating may be countered by a need for more venting for dehumidification purposes. To reduce heat loss during times of low outdoor temperatures, thermal curtains are widely used in greenhouses to retain thermal energy near the plants and prevent radiative heat losses to the outside. Among passive techniques, thermal curtains are one of the most practical and appropriate methods of reducing consumption of heat [10]. Another passive technique, utilizing a sensible or latent thermal energy storage 3

19 material on the interior of the greenhouse, has also been investigated by various authors [11,12,13]. Energy conservation potential, by adjusting temperature set-points based on outdoor temperatures and available solar radiation, has also been shown [5]. Energy savings without any reduction in crop yield were found by controlling mean air temperature; reducing the set point during cold outdoor conditions and increasing afterwards when favourable conditions are present. Also, the appropriate placement of heating pipes within the greenhouse has been shown to play an important role in reducing energy consumption and ensuring homogeneous temperature distribution [14,15,16]. Although several design and operational strategies for energy conservation are available to greenhouse operators, they are implemented to varying extents and greenhouses are still heavily dependent on fossil fuels. In an effort towards a sustainable energy supply, researchers in the Netherlands state that simple measures like installing a moveable energy screen in traditional greenhouses or improving existing designs on a small scale are not enough [17]. Based on this, closed greenhouse systems have been in development since the late 1990 s to conserve energy [18]. Instead of using ventilation strategies during the summer months to release heat and dehumidify the greenhouse, active cooling and dehumidification systems are used. Excess solar energy collected during the summer months is stored via some form of seasonal thermal energy storage (STES), typically aquifer thermal energy storage (ATES), and re-used in the winter to heat the greenhouse [19]. A general schematic diagram of the closed greenhouse energy flow is shown in Figure 1B. In practice, semi-closed systems are most common where the cooling system is designed to meet a base load and peak demands are still met with ventilation strategies. Alternatively, in closed greenhouses without an STES system, heat recovery ventilation can be utilized where the warm greenhouse air being removed can heat the colder incoming air from the outside, reducing the overall demand [20]. Installed systems in the Netherlands with seasonal storage have shown that heating energy savings in the range of 20 to 60% are possible compared to the traditional open greenhouse [21,22,23]. As carbon dioxide is injected into the greenhouse to promote crop growth, reduction or elimination of natural ventilation leads to a more consistent and elevated indoor CO 2 concentration by minimizing losses to the outdoor environment. This has shown to increase crop production by 10-20% [22,23]. The above advantages have also been accompanied by significant decreases in pesticide use and water consumption for irrigation. In the Belgian climate, Coomans et al. determined that heat recovery ventilation strategies in a semi-closed greenhouse reduced annual heating demand by up to 28% [20]. They also found it to be most effective during the spring and fall seasons. In colder climates, winter heating demands are significantly increased and closed greenhouse installations are limited. Experiments in semi-closed greenhouses with heat recovery ventilation in Finland showed a decrease in the summer heating usage by 35-50%, but no significant reductions throughout the rest of the year [24]. Wong et al. assessed the closed greenhouse with seasonal storage for the Canadian climate setting and found that annual greenhouse gas emissions could be reduced by up to 86% [1]. However, the authors stated it was difficult to obtain directly applicable information in Canada in support of the reported study. Yildiz et al. then compared conventional, semi-closed and closed greenhouse systems equipped with air-source heat pumps throughout Canada [25]. They 4

20 determined the semi-closed systems provided considerable savings in both energy use and water consumption over the conventional greenhouse. For systems with seasonal storage, an interesting characteristic to consider is the surplus energy ratio (SER). The SER is the annual ratio between excess heat during the summer months and heating demand during the winter months [26]. In Sweden, Vadiee and Martin determined an ideally closed greenhouse has an SER ratio of about three [26]. The same authors also found the most influential factor on payback period of a closed greenhouse system with seasonal storage is whether the system is designed for peak or base load [27]. (A) Open Greenhouse (B) Closed Greenhouse Figure 1 General Schematic Diagram of Energy Flow in Open and Closed Greenhouse This paper aims to expand on past analyses of closed greenhouse systems in cold climates. The Canadian landscape has been chosen as it contains approximately 2,400 hectares of greenhouse area [28]. Data obtained from greenhouse operators has shown that in excess of 500,000 cubic metres of natural gas is used annually per hectare for heating purposes. Where past studies have evaluated small-scale greenhouses or have not focused on monthly 5

21 heating and cooling data, this paper specifically assesses the monthly demands and surplus energy ratio with varying cover materials for a 0.4 hectare greenhouse. The interior microclimate of the greenhouse is modelled using TRNSYS software and validated with natural gas usage data from a reference greenhouse. The effect of location and cover material on the SER and the potential for heat recovery ventilation is assessed for the most concentrated greenhouse areas in the country. 2.0 Greenhouse Model The software program TRNSYS, a Transient System Simulation Program, was utilized to simulate the greenhouse microclimate. TRNSYS is a complete and extensible simulation environment for the transient simulation of systems, including multi-zone buildings [29]. To model the reference greenhouse a Type 56 Multi-Zone Building component was used. A 3-dimensional rendering of the greenhouse was created. The structural and thermal properties were assigned utilizing the TRNSYS sub-program TRNBuild. Climate data is fed into the TRNSYS simulation environment by an external weather module. Based on the location, orientation and geometry of the structure, global solar radiation incident on each of the external facades is calculated at each time step by internal modules. The modelled Venlo-type greenhouse has a plan area of 4,000 m 2, gutter height of 5.5 metres (m), 10 bays each with a width of 5 m, and a roof slope of 25. A portion of the greenhouse is shown in Figure 3. The structure was created with a series of lower and upper thermal zones to simulate conditions within and above the crop. The lower zone extends to a height of 3.5 m. The greenhouse exterior was approximately 96% glazed to account for shading from construction elements. The glazing material properties were chosen to represent those of double polyethylene (PE) cover material and an overall average heat transfer coefficient of 3.2 W/ m 2 C was chosen, which is within the range of values reported by others [7]. The coefficient is not constant and is calculated by the TYPE 56 component at each time step based on the outside climate parameters and interior conditions [30]. Figure 2 Components of TRNSYS Simulation 6

22 2.1 Energy and Mass Balance The greenhouse micro-climate is a dynamic environment influenced by the outdoor conditions, internal control mechanisms and indoor factors [10]. Energy and mass balance of the greenhouse interior components is essential to appropriately describe the environment. It is typically comprised of five major components: growing medium, floor, crop, greenhouse cover, and indoor air, with the control volume ending at the outdoor ambient air [10,31]. The simulation conducted herein focuses on the final four components listed. The thermal capacity of each lower node has been set to simulate the presence of a crop. Discussions with greenhouse operators revealed crop density is initially very small at the time of planting and can reach up to 10 kg/ m 2 when fully grown. Increasing the thermal capacity of the lower nodes throughout the year was not possible in the structure and a constant crop density of 6 kg/ m 2 has been utilized. The thermal properties of the crop are assumed to be similar to those of water [11]. A sensible energy balance was carried out for the air in each thermal zone considering gains from surfaces of the zone, infiltration, ventilation, coupling air flows with adjacent zones and internal gains. The sensible energy flux can be described as follows [30]: Q sens,i = Q surf,i + Q inf,i + Q vent,i + Q g,c,i + Q cplg,i (1) Where: Q sens,i = Sensible Energy Flux of Zone [kj/hr] Q surf,i = Convective Gain from Surfaces Q inf,i = Infiltration Gains Q vent,i = Ventilation Gains Q g,c,i = Internal Convective Gains = Gains from Coupling Air Flows from Adjacent Zones Q cplg,i The energy flux for each particular surface is calculated considering combined convective and radiative energy fluxes. The solar radiation flux is calculated for external surfaces where internal surfaces also include long-wave radiation exchange between internal objects and adjacent walls. Infiltration, ventilation and coupling gains are dependent on user-defined air movement rates and temperature differences between environments. These gains can be defined for each zone by the following: Q x = V ρ C p (T x T i ) (2) Where x represents the particular gain, V is the defined air flow rate, ρ is the density of air, C p is the specific heat capacity of air, and T x and T i are the temperatures of the incoming air and zone air at the previous time step, respectively. The temperatures of the infiltration and ventilation gains are set to outdoor ambient conditions. Infiltration was set at 0.5 air changes per hour (ACH) and ventilation was set at 60 ACH when active [32,33]. Coupling air flows between lower zones begins at 1 ACH and gradually decreases to simulate the stagnation of horizontal air movement as the crop increases in size, as shown in Figure 3. Internal gains in the model encompass 7

23 energy that is convected and radiated from the outside surface of the hot water and steam heating pipes. Indoor air velocity was assumed to be 0.15 m/s based on estimates by others [15,34]. The latent energy flux for each thermal zone is determined by an effective capacitance humidity model. Similar to sensible energy flux, the model considers the latent energy gained or lost by the air in the zone due to infiltration, ventilation, coupling air flows and internal gains. The latent energy flux is calculated at the end of each time step based on the following [30]: Q lat,i = h v [m inf,i (h a h req,i ) + m vent (h vent h req,i ) + (3) W g,i + Σ surfj-k m cplg (h j h i ) M eff (h req,i h i, t- t) / t)] Where Q lat,i = Latent Energy Flux of Zone [kj/hr] h v = Heat of Vapourization of Water [kj/kg] m = Air Mass Flow Rate [kg/m 3 ] h = Humidity Ratio [kg water / kg air ] W g,i = Internal Humidity Gain [kg water /hr] M eff = Effective Moisture Capacitance of Zone [kg] t = Length of Timestep Subscripts a Outdoor Ambient vent Ventilation inf Infiltration req Required cplg Coupling It is known that crop evapotranspiration, that is transpiration from the crop leaves and evaporation from the growing medium, plays an important role in the energy balance of the greenhouse microclimate [10,31]. Crop evapotranspiration was simulated in the model via an internal humidity gain. The humidity gain is gradually increased after planting to account for increases in crop size and solar radiation and reaches a maximum of 25 grams of water /hour /m 2 during the summer months when solar radiation is highest and the crop is nearing maximum height. 8

24 Figure 3 Crop (Lower) Zone Coupling Air Flow 2.2 Model Controls Day and night set point temperatures are set to those of the reference greenhouse. Ventilation is active when the interior temperature exceeds 25 C or relative humidity exceeds 85%. Thermal curtains are closed when global solar radiation is less than 5 W /m 2. Both steam and hot water boilers are utilized in the model with boiler efficiency considered to be 75%. The hot water boiler feeds a stratified water storage tank. Steam and hot water piping systems are modelled, with the hot water system located in the lower nodes and the steam system located in the upper nodes. Morning pre-heating was active between 3 and 6 am. The total heat transfer from the hot water and steam piping systems to the interior of the greenhouse was monitored. A high-pressure fogger is used for humidity control and set-points are those of the reference greenhouse. 3.0 Reference Greenhouse The reference greenhouse chosen for this project is an approximately 8.1 hectare venlo-type greenhouse with double PE cover. The greenhouse is located in Leamington, Ontario and is used for pepper cultivation of various varieties. The heating system consists of both steam and hot water piping systems, with the hot water system located near the floor of the greenhouse and the steam system located near the greenhouse roof. The hot water system is the primary heat source. The steam system is used to remove condensation from the greenhouse cover in the early morning hours and also during times of peak heating demand. Indoor day and night set-point temperatures are typically 23 C and 22 C, respectively. Relative humidity is generally maintained between 75 and 85%. Thermal curtains are utilized and closed at night during the winter months. During the summer months the curtains are not utilized as outdoor ambient temperatures during the night are generally above 15 C and heat retention is not necessary. To avoid large and sudden changes in indoor temperature, the temperature of the greenhouse is raised approximately 1 C / hour in 9

25 the early morning hours to reach the daytime set-point prior to the sun rising. This morning pre-heating activates the plants out of the cool night temperatures and takes place throughout the growing season. Six years of energy usage data from 2009 to 2015 was obtained to validate the model. Natural gas is the primary fuel for heating purposes, however due to shortages during the coldest of winter nights, coal and heavy oil are also used for supplemental heating. The monthly energy usage for heating purposes is shown in Figure 4. The average annual energy usage is approximately 7,900 Gigajoules (GJ) per 0.4 hectares. Planting typically takes place in the first week of January and the crop is terminated in mid-november. The temperature in the greenhouse is held at 5 C after this time. This is why the heating energy usage for November and December is low in comparison to the decreasing ambient temperatures during these months. 4.0 Results and Discussion 4.1 Reference Greenhouse The greenhouse simulation was performed over a one-year period beginning on January 1 with time steps equal to 1 hour. Weather data for Detroit, Michigan was utilized due to its close proximity to Southwestern Ontario. The typical meteorological year (TMY2) data set is utilized and represents typical conditions based on data from the National Solar Radiation Data Base (NSRDB) and is produced by the National Renewable Energy Laboratory (NREL) ( [35]. The required energy input for both the steam and hot water boiler are calculated and compared to the average monthly energy usage data from the reference greenhouse. The results are shown in Figure 4. As can be seen, the model adequately simulates the reference greenhouse energy usage. The total annual energy demand was about 7,700 GJ for the 0.4 hectare greenhouse, a deviation of about 3% from the reference data. The monthly heating energy demand and energy removed due to ventilation for the simulated reference greenhouse are shown in Figure 5. The heating energy demand, that is the heat delivered by the hot water and steam piping systems, was approximately 4,700 GJ. The model was also run over a two-year period and energy usage was 0.1% greater in the second year, likely due to initial start-up of the heating system at the beginning of the simulation. The interior greenhouse temperature generally peaked above 30 C on summer days and rose above 40 C on 6 dates; July 3 rd, 5 th, 6 th, 8 th, 11 th and 12 th. Ventilation kept the relative humidity in the greenhouse generally below 85%. Energy removed from the greenhouse due to ventilation was about 4,000 GJ. Based on this, the surplus energy ratio (SER), as defined in equation 4, for the reference open greenhouse would be approximately SER = Annual Cooling Demand (4) Annual Heating Demand 10

26 Energy Demand (GJ) Energy Use (GJ) Actual Energy Use Modelled Energy Use Month Figure 4 Actual and Modelled Energy Usage for the Reference Greenhouse 1, , Heating Demand Ventilation Cooling Month Figure 5 Monthly Heating Demand and Ventilation Cooling for Simulated Open Reference Greenhouse 4.2 Closed Greenhouse To simulate the closed greenhouse, ventilation was not utilized and the cooling demand was assessed based on a set point temperature of 25 C. An ideally constructed closed greenhouse was considered and infiltration was reduced to 0 ACH. Both sensible energy demand and latent energy demand for humidity control were considered. Results of the simulation showed that the heating demand decreased to approximately 2,800 GJ. The cooling 11

27 Energy Demand (GJ) demand of the greenhouse increased to approximately 5,300 GJ giving an SER of about 1.9. Additional dehumidification was necessary as the inside relative humidity regularly exceeded 85% during the summer months. This is due to the decreased interior temperature in relation to the open greenhouse and inability to dehumidify via exchange with outdoor air. This occurrence is consistent with observations by others [1,36]. The monthly heating and cooling demands of the closed greenhouse are shown in Figure 6. By comparing Figures 5 and 6 we can see that no significant decrease in heating demand occurs between the months of May through August. This can be attributed to morning pre-heating of the crop, which occurs throughout the summer months regardless of the interior temperature. Throughout the fall, winter and spring seasons daytime high ambient temperatures are generally lower than the greenhouse set-point temperature. The cooling demand during these seasons is also focused around the daytime hours. Heat recovery ventilation would provide the opportunity to cool and dehumidify the interior air while warming the colder outside air before entering the greenhouse, thus reducing the overall heating demand during these seasons. This would be most effective in March, April, September and October where considerable cooling demands are present. During the summer months the daytime ambient temperature is generally above the indoor set-point and warming of the air before entering the greenhouse is not necessary. 1,200 1, Heating Demand Cooling Demand Month Figure 6 Monthly Heating and Cooling Demand for Closed Reference Greenhouse The greenhouse cover material plays an important role in heating and cooling demands. To assess the effects of different covering materials, the annual energy demands have also been assessed for single and double glass cover materials. For single and double glass overall heat transfer coefficients of 5.7 and 1.4 W/ m 2 C, respectively, were 12

28 Annual Energy Demand (GJ) utilized [37]. The results of this assessment are shown in Figure 7 along with the demands for the reference cover material of 3.2 W/ m 2 C, representative of double PE. The cooling demand increases by approximately 23% for the double glass covering material to 6,600 GJ, while the demand for single glass showed a slight increase to about 5,400 GJ. The heating demand steadily increases with the increase of cover heat transfer coefficient. The SER for U- values of 1.4, 3.2 and 5.7 W/ m 2 C are 3.4, 1.9 and 1.5, respectively. 7,000 6,578 6,000 5,337 5,447 5,000 4,000 3,627 3,000 2,000 1,913 2,854 Heating Demand Cooling Demand 1, U-Value (W/ m2/ C) Figure 7 Annual Heating and Cooling Demand for Closed Greenhouse with Differing Cover Materials 4.3 Closed Greenhouse in Different Canadian Settings The three most concentrated greenhouse areas in Canada are Ontario, British Columbia and Quebec. The locations of Montreal, Quebec and Vancouver, British Columbia, both located in the Southern portion of their respective provinces, were chosen as simulation sites. Meteonorm climate data published by Meteotest ( was utilized for these locations [35]. Montreal typically has colder winters and comparable summers to Southern Ontario, whereas the climate in Vancouver is much milder for both seasons. The temperature data used in the simulation for each of the locations is shown in Figure 8. The hourly data for Detroit, Michigan is shown. However, the hourly data for the other two locations has been removed for clarity and is represented by a polynomial trend line. Identical controls and set points were utilized for each of the chosen locations. The annual demands in relation to cover properties are shown in Figure 9. The heating and cooling demands make practical sense in relation to the temperature data. Montreal, with significantly colder winters than the other two locations has an appropriately higher heating demand. Similarly Detroit, representing the Southern Ontario climate, 13

29 Temperature ( C) has the warmest summers and appropriately the largest cooling demand of the three locations. Table 1 presents the annual demands and SER ratios for the various conditions. As can be seen a surplus energy ratio of at least 1 is achieved in all locations. Heat recovery ventilation potential was observed to be similar to that of Southwestern Ontario. The spring and fall seasons offered the greatest potential to significantly reduce the heating demand due to significant ventilation that takes place while outdoor temperatures are below the interior set-point. It can be seen that for a U-Value of 5.7 W/ m 2 C the cooling load is only slightly less than that for a U-Value of 3.2 W/ m 2 C and is actually higher for the Southwestern Ontario climate. This is attributed to greater solar radiation transmission through the single glass cover material Vancouver, BC Montreal, QC Detroit, MI Poly. (Vancouver, BC) Poly. (Montreal, QC) Poly. (Detroit, MI) Hour of Year Figure 8 Temperature Data 14

30 Annual Energy Demand (GJ) Detroit, MI - Heating Detroit, MI - Cooling Montreal, QC - Heating Montreal, QC - Cooling 3000 Vancouver, BC - Heating Vancouver, BC - Cooling U-Value (W/ m 2 / C) Figure 9 Closed Greenhouse Annual Heating and Cooling Demands for Differing Locations and Cover Materials Table 1 - Annual Heating and Cooling Demands for Closed Greenhouse with Differing Locations and Cover Materials (Heating (GJ), Cooling (GJ), SER) Cover U-Value (W/ m 2 / C) Location Heating / Cooling/ SER Heating / Cooling/ SER Heating / Cooling/ SER Detroit, MI 1,910/ 6,580/ 3.4 2,850/ 5,340/ 1.9 3,630/ 5,450/ 1.5 Montreal, QC 2,430/ 6,400/ 2.6 3,590/ 5,150/ 1.4 4,600/ 4,800/ 1.0 Vancouver, BC 1,940/ 5,970/ 3.1 2,850/ 4,710/ 1.7 3,710/ 4,520/ Discussion An SER ratio of three for a closed greenhouse has been reported in other studies [23,26]. This is in agreement with the results of this study. The day and night set point temperatures of 23 C and 22 C used herein are suitable for this particular grower. However, lower set-point temperatures may be more representative of a broader range of greenhouse operations. A decrease in the set point temperatures and in turn the heating energy demand would lead to an overall increase in the SER, especially in the cold climates studied here. 15

31 In assessing the potential of a closed greenhouse system, an SER of 1 is necessary if the entire heating demand is to be met with heat removed and stored from the summer months. As can be seen it appears this can be achieved at all three locations with any of the cover materials studied. However, in addition to a sufficiently large cooling capacity, a seasonal thermal energy storage system of sufficient capacity must be feasible to fully close the greenhouse. The high-insulating double glass cover material, in addition to offering the largest SER, also significantly decreases the annual heating demand and in turn the necessary storage capacity. The heating energy savings observed with the double glass cover are generally within the range of values reported by others [9]. With a high-insulating cover material and the inability of the closed greenhouse to mix indoor air with outdoor, the greenhouse interior becomes less affected by the outdoor conditions and the environment is more easily controlled. Hence it can be concluded that if a fully closed greenhouse is desirable, a cover material with high-insulating properties should be utilized, provided light transmission is not sacrificed. This is consistent with recommendations for energy efficient greenhouse design [21,38]. As an SER ratio of three provides far more cooling capacity than necessary for a closed greenhouse design, a semiclosed greenhouse would allow the cooling capacity to be technically and economically optimized at a much lower base cooling load. In this case, natural ventilation strategies would be utilized to cover peak cooling loads. The amount to which the greenhouse could be opened is dependent on the efficiency of the cooling, dehumidification and seasonal storage systems. Reduction in fossil fuel consumption must be weighed against the increase in electricity usage to run circulation pumps, heat pump(s), and humidity control systems [22]. Furthermore, the energy cost savings need to be assessed in relation to capital investment costs while also considering the economic benefits of potential increases in crop production. An overall assessment with these factors can determine if an optimal system lies with seasonal storage or a heat recovery ventilation system. 5.0 Sensitivity Analysis A sensitivity analysis was carried out on the greenhouse model to determine the relative impact of altering key model parameters. The analysis observed the heating demand for the month of January and the percent change relative to the initial baseline demand. The results are shown in Figure 10. It can be observed that altering the infiltration rate and indoor set-point temperatures showed the greatest change in the monthly demand. As each of these parameters directly affect the required heating demand the results are considered reasonable. The remaining parameters showed small changes to the heating demand. Overall, the analysis gives a satisfactory level of confidence in the stability of the model. It should be noted that changes to certain parameters are not linear and are based on dynamic operational controls of the system. For example, as the infiltration rate decreases the amount of cold outside air naturally entering the building decreases. However, this leads to increased indoor temperatures and humidity and more ventilation is then required, bringing in greater amounts of cold outside air through ventilation. 16

32 Figure 10 Sensitivity Analysis Heating Demand for Month of January (% Change from Baseline Value) For the simulation of the greenhouse heating system encompassing the steam and hot water boilers, piping systems, water storage tank and system controls, there were several parameters involved. These parameters were estimated based on conversations with greenhouse operators and available information. It is realized that changes to system parameters can have direct and significant effects on system energy use. Among these, the boiler efficiency parameter directly correlates to the amount of energy required to produce a unit amount of useful heat. Overall system efficiency can be defined as the useful thermal energy delivered to the greenhouse divided by the energy input to the boilers: Ƞ system = Useful Thermal Energy Delivered to Greenhouse (5) Energy Input to Boilers Figure 11 presents the monthly system efficiency for the reference greenhouse. It can be seen that efficiency ranges from approximately 66% during the month of January to 48% during the summer months. The decrease in efficiency can be attributed to the lower heating demand during the summer months, which resulted in more on/off operation of the boilers. However, this cannot be confirmed. Furthermore, heating systems vary between growers and this efficiency curve will not be consistent. It is therefore concluded for future work it may be more appropriate to simply assess the required heating demand of the greenhouse at each time step based on the interior set-point temperature, rather than simulating the operation of the entire heating system. The heating demand profile can be compared to the obtained grower energy usage data for comparison. 17

33 Energy (GJ) System Efficiency (%) Heating Demand Energy Use System Efficiency Month Figure 11 Overall Heating System Efficiency 6.0 Conclusion This paper provides an assessment of the closed greenhouse system in a cold climate considering different Canadian settings. The interior microclimate of a 0.4 hectare greenhouse has been modelled using TRNSYS software and validated with natural gas usage data from a regional grower. The following conclusions can be made: 1) An SER ratio ranging from 1.5 to 3.4 was observed for the Southwestern Ontario location considering single glass, double polyethylene, and double glass cover materials, respectively. The SER ratio for the locations of Montreal, Quebec and Vancouver, British Columbia was found to range between 1.0 and 3.1. In conclusion, the annual cooling demand is equal to or greater than the heating demand for all locations and a fully closed greenhouse is possible in these cold climate conditions. 2) The use of a high-insulating double glass cover material would likely be most suitable for a closed greenhouse system with seasonal storage. This is due to the significant reduction in annual heating demand and in turn the necessary seasonal storage capacity. 3) Heat recovery ventilation has potential to reduce the heating demand during the fall, winter and spring seasons. This is due to significant ventilation that takes place during these seasons while ambient temperatures are below the interior set-point. 4) A semi-closed greenhouse would allow the cooling capacity to be optimized at a much lower base cooling load. The amount to open the greenhouse, as well as the options of seasonal storage or heat recovery ventilation, are site dependent. The optimal design will need to be assessed considering the reduction in fossil fuel usage, increase in electricity usage to run the active components of the system, as well as the anticipated increase in crop production. 18

34 References [1] Wong, B., McClung, L., McClenahan, D., Snijders, A., Thornton, J. (2011). The Application of Aquifer Thermal Energy Storage in the Canadian Greenhouse Industry, Acta Hort. 893, [2] Fernandez, M.D., Rodrıguez, M.R., Maseda, F., Velo, R., Gonzalez, M.A. (2005). Modelling the Transient Thermal Behaviour of Sand Substrate heated by Electric Cables, Biosystems Engineering 90 (2), [3] Hemming, S., Kempkes, F.L.K., Janse, J. (2012). New Greenhouse Concept with High Insulating Double Glass and New Climate Control Strategies Modelling and First Results from a Cucumber Experiment, Acta. Hort. 952, [4] Sturm, B., Maier, M., Royapoor, M., Joyce, S. (2014). Dependency of production planning on availability of thermal energy in commercial greenhouses-a case study in Germany, Applied Thermal Engineering 71, [5] Spanomitsios, G.K. (2001) Temperature control and energy conservation in a plastic greenhouse, J. agric. Engng. Res. 80(3), [6] Ozgener, O., Hepbasli, A. (2005). Experimental investigation of the performance of a solar-assisted groundsource heat pump system for greenhouse heating, Int. J. Energy Res. 29, [7] Zhang, Y., Gauthier, L., Halleux, D., Dansereau, B., Gosselin, A. (1996). Effect of covering materials on energy consumption and greenhouse microclimate, Agricultural and Forest Meteorology 82, [8] Hemming, S., Kempkes, F.L.K., Mohammadkhani, V. (2009). New Glass Coatings for High Insulating Greenhouses Without Light Losses Energy Saving, Crop Production and Economic Potentials. Acta Hort. 893, [9] Swinkels, G.L.A.M., Sonneveld, P.J., Bot, G.P.A. (2001). Improvement of Greenhouse Insulation with Restricted Transmission Loss through Zigzag Covering Material, J. agric. Engng Res. 79(1), [10] Sethi, V.P., Sumathy, K., Lee, C., Pal, D.S. (2013). Thermal modeling aspects of solar greenhouse microclimate control: A review on heating technologies, Solar Energy 96, [11] Din, M., Tiwari, G.N., Ghosal, M.K., Srivastava, N.S.L., Imran Khan, Md., Sodha, M.S. (2003). Effect of thermal storage on the performance of greenhouse, Int. J. Energy Res. 27, [12] Gupta, A., Tiwari, G.N. (2002). Computer Model and Its Validation for Prediction of Storage Effect of Water Mass in a Greenhouse: A Transient Analysis, Energy Conversion and Management 43, [13] Beyhan, B., Paksoy, H., Daşgan, Y. (2013). Root Zone Temperature Control with Thermal Energy Storage in Phase Change Materials for Soilless Greenhouse Applications, Energy Conversion and Management 74, [14] Hao, X., Borhan, M.S., Papadopoulos, A.P., Zheng, J., Khosla, S. (2007). Effects of Heat Placement on Energy Consumption, Microclimate, Plant Growth and Fruit Yield of Greenhouse Tomatoes Grown on Raised-Troughs, Acta. Hort. 761, [15] Kempkes, F.L.K., Van de Braak, N.J., Bakker, J.C. (2000). Effect of Heating System Position on Vertical Distribution of Crop Temperature and Transpiration in Greenhouse Tomatoes, J. agric. Engng Res. 75, [16] Winspear, K.W. (1978). Vertical Temperature Gradients and Greenhouse Energy Economy, Acta Hort. 76, [17] Waaijenberg, D., Hemming, S., Campen, J.B. (2005). The Solar Greenhouse: a Highly Insulated Greenhouse Design with an Inflated Roof System with PVDF or ETFE Membranes, Acta. Hort. 691, [18] Hoffman J., Loeber A. (2016). Exploring the Micro-politics in Transitions from a Practice Perspective: The Case of Greenhouse Innovation in the Netherlands, Journal of Environmental Policy & Planning, 18 (5), [19] Qian T., Dieleman J.A., Elings A., De Gelder A., Marcelis L.F.M. (2015) Response of Tomato Crop Growth and Development to a Vertical Temperature Gradient in a Semi-Closed Greenhouse, Journal of Horticultural Science & Biotechnology 90(5), [20] Coomans M., Allaerts K, Wittemans L, Pinxteren D. (2013). Monitoring and energetic performance of two similar semi-closed greenhouse ventilation systems, Energy Conversion and Management 76, [21] Bot, G., van de Braak, N., Challa, H., Hemming, S., Rieswijk, T., v. Straten, G., Verlodt, I. (2005). The Solar Greenhouse: The State of the Art in Energy Saving and Sustainable Energy Supply, Acta. Hort. 691,

35 [22] Opdam, J., Schoonderbeek, G., Heller, E., de Gelder, A. (2005). Closed Greenhouse: a Starting Point for Sustainable Entrepreneurship in Horticulture, Acta Hort. 691, [23] Marcelis, L., Buwalda, F., Dieleman, J., Dueck, T., Eilings, A., de Gelder, A., Hemming, S., Kempkes, F., Li, T., van Noort, F., de Visser, P. (2014). Innovations in Crop Production: A Matter of Physiology and Technology, Acta. Hort. 1037, [24] Kaukoranta T., Näkkilä J., Särkkä L., Jokinen K. (2014). Effects of Lighting, Semi-Closed Greenhouse and Split-Root Fertigation on Energy Use and CO 2 Emissions in High Latitude Cucumber Growing, Agricultural and Food Science 23, [25] Yıldız I., Yue J., Yıldız A.C. (2015) Air Source Heat Pump Performance in Open, Semi-closed, and Closed Greenhouse Systems in British Columbia; the Canadian Maritimes; Ontario; Canadian Prairies; Quebec and Labrador. In: Dincer I.,Ozgur Colpan C., Kizilkan O., Akif Ezan M., Progress in Clean Energy, Volume 1:Analysis and Modelling., pp Springer, Switzerland. [26] Vadiee, A., Martin, V. (2013). Energy Analysis and Thermoeconomic Assessment of the Closed Greenhouse The Largest Commercial Solar Building, Applied Energy 102, [27] Vadiee, A., Martin, V. (2013). Thermal Energy Storage Strategies for Effective Closed Greenhouse Design, Applied Energy 109, [28] Statistics Canada (2016) Table , Estimates of Greenhouse Total Area and Months of Operation [WWW document]. URL [Accessed on 23 February 2017] [29] Solar Energy Laboratory (SEL), University of Wisconsin-Madison (2014). TRNSYS 17 Manual, Volume 1, pp.8. SEL, Madison. [30] Solar Energy Laboratory (SEL), University of Wisconsin-Madison (2014). TRNSYS 17 Manual, Volume 5, pp.75-76, 147, , SEL, Madison. [31] Cooper, P.I., Fuller, R.J. (1983). A Transient Model of the Interaction Between Crop, Environment, and Greenhouse Structure for Predicting Crop Yield and Energy Consumption, J. Agric. Engng. Res. 28, [32] Lee W.F. (2010) Cooling Capacity of Semi-Closed Greenhouses, Master s Thesis, The Ohio State University, Columbus, Ohio. pp. 62, 82 [33] Aldrich R. A., Bartok J. W. (1989) Greenhouse Engineering. Northeast Regional Agricultural Engineering Service, Ithaca. [34] Abdel-Ghany, A.M., Kozai, T. (2006). On the determination of the Overall Heat Transmission Coefficient and Soil Heat Flux for a Fog Cooled, Naturally Ventilated Greenhouse: Analysis of Radiation and Convection Heat Transfer, Energy Conversion and Management 47, [35] Solar Energy Laboratory (SEL), University of Wisconsin-Madison (2011). TRNSYS 17Manual, Volume 8, pp. 7, 17. SEL, Madison. [36] Campen, J.B., Kempkes, F.L.K. (2011). Climatic Evaluation of Semi-Closed Greenhouses, Acta Hort. 893, [37] ASHRAE (2013) Chapter 15, Fenestration. In: 2013 ASHRAE Handbook, Fundamentals, Table 4 - U-Factors for Various Fenestration Products. ASHRAE, Atlanta. [38] Dieleman, J.A., Hemming, S. (2011). Energy Saving: from Engineering to Crop Management, Acta Hort. 893,

36 CHAPTER III Potential for Large-Scale Solar Collector System to Offset Carbon- Based Heating in Ontario Greenhouse Sector Lucas Semple +, Rupp Carriveau +, David S-K. Ting ± + Department of Civil and Environmental Engineering, University of Windsor, Windsor, Ontario, Canada ± Department of Mechanical Automotive and Materials Engineering, University of Windsor, Windsor, Ontario, Canada 1.0 Background Climate change and air quality continue to be significant challenges, with wide ranging effects to social, environmental and economic well-being [1]. Extreme weather events continue to have grave consequences for human health and infrastructure. As people s reliance on infrastructure increases, when it is damaged and destroyed the effects are more widely felt. In Canada, these wide spread effects have clearly been seen. The years of 2009 to 2012 saw record-high levels of insured losses from natural disasters, with claims near or above $1 Billion [2]. This was followed by a historic $3.2 Billion in losses in 2013 as a result of flooding in Alberta and Toronto. For comparison, total insured losses averaged $400 Million for the 25-year period between 1983 and The need for progress towards a low-emission economy is increasingly better understood. To avoid dangerous climate change, greenhouses gas emissions need to stabilize in 5~10 years and approach zero by the second half of the century [3]. In Ontario, overall provincial emissions reduced by approximately 5.9% between 1990 and 2012, however emissions from transportation and buildings increased by 8% and 2%, respectively, and are currently the most significant contributors to Ontario s Emissions outside of electricity [4]. Accounting for less than 1 per cent of global emissions, Ontario is still among the largest per capita emitters of greenhouses gases in the world [5]. With over 1,000 hectares of greenhouses as of 2015, building energy demands are a major factor in the Ontario greenhouse sector. For a greenhouse to maintain an adequate indoor temperature throughout the year, space heating demands can account for up to 90% of the total seasonal energy demand [6]. This heating demand is primarily met by natural gas infrastructure throughout the province. With current constraints on the natural gas and electricity grids threatening to impede expansion and Ontario electricity prices continuing to rise, a low-emission innovative energy solution is needed to meet the energy demands of this important sector and contribute to reducing overall greenhouse gas emissions. 21

37 2.0 Introduction The indoor temperature of a greenhouse strongly influences the rate of development, fruit colour, and balance between vegetative growth and fruit development [7]. In Ontario, bell peppers, tomatoes and cucumbers are the main greenhouse production crops and have optimum 24-hour indoor temperatures of generally between 18 and 22 o C. In order to facilitate year-round crop production, agricultural greenhouses have a large heating demand during the winter months. Natural gas fired boilers are utilized in the majority of greenhouses in Ontario and some have the ability to combust heavy oil for supplemental use. During the summer months overheating is controlled by ventilation techniques using vents in the greenhouse cover where indoor air is mixed with outdoor. A review of available greenhouse heating technologies has been carried out by others [8]. The majority of non-fossil fuel dependent techniques have been installed in relatively small-scale greenhouses. The potential of flat plate solar collectors to improve the indoor temperature of a greenhouse has shown promise provided sufficient solar radiation is available [9,10]. However, one of the barriers to utilizing solar energy for space heating is the misalignment between available solar radiation and building heating demand in the greenhouse sector and elsewhere. This renders a solar collector system unviable without incorporating some form of seasonal thermal energy storage [11]. Interestingly, in addition to the ventilation techniques utilized during the summer months to cool the greenhouse, heat is supplied in the hours prior to sunrise to avoid sharp changes in indoor air temperature between the night and day. The indoor air temperature is recommended to be raised by 1 o C/ hour to achieve the indoor daytime set point temperature approximately ½ hour prior to sunrise [7]. After analyzing monthly natural gas usage for twelve growing years from three different greenhouse operators in Ontario, it was observed that over 20% of the total annual natural gas usage occurred between May and September. These are typically the warmest months of the year and receive the highest amount of solar radiation. In addition, the year-round heating demand present in greenhouses is a key factor to maximizing utilization of a large-scale collector system. Based on this, a potential exists for a large-scale solar collector system to offset greenhouse carbon-based heating during the summer months. Large-scale solar collector systems for space and water heating applications were first installed in Europe in the late 1970 s [12]. As of 2007, approximately 200,000 m 2 of solar collectors were installed in Europe as part of large-scale systems greater than 500 m 2 in size. However, this accounted for only 1% of the total installations in Europe at the time with the vast majority being small-scale systems, typically 2~30 m 2 in size, as part of solar domestic hot water systems. The majority of large-scale systems have been employed in block and district heating networks, however, systems have been utilized elsewhere and have potential in buildings with a large year-round heating demand [13]. Denmark as of 2013, with approximately 386,000 m 2 of installed solar collectors in large-scale applications, hosts 9 of the 10 largest solar heating plants in Europe [14]. Systems have been designed since the early 2000 s to handle both heating and cooling loads utilizing heat-driven cooling devices. Large-scale solar collector systems have been less prevalent in North America. One large-scale system equipped with seasonal energy storage was commissioned in 2007 in Okotoks, Alberta. After 5 years of operation, the system was able to provide 97% of the space heating needs for a community of 52 houses [15]. 22

38 Large-scale solar heating systems have two major applications. The first and most common are systems equipped with diurnal (short-term) storage, usually consisting of an insulated steel tank for storing hot water. Systems with short-term storage are generally designed to meet 10~20% of the total annual load [16,17]. The majority of largescale solar collector systems are designed in this manner and their intent is to cover the summer hot water and space heating load [12]. The second application are systems equipped with long-term or seasonal storage, where solar radiation captured during the summer months can be stored and utilized during the colder months when heating demands are greatest. These systems are usually designed to meet 50~70 % of the total annual load. However, initial investment can be double that of a system with short-term storage due to the high initial costs of the seasonal storage component [17]. In this study, a transient simulation is carried out utilizing TRNSYS software to model the indoor greenhouse microclimate and determine the heating demand profile for a greenhouse in Southern Ontario, an area with the highest density of greenhouses in North America. The heating demand profile is validated with actual natural gas usage data from Ontario greenhouses. Finally, a large-scale solar thermal collector system designed to meet the summer heating demand is incorporated and the reduction in natural gas and carbon emissions are explored. An economic assessment of system installation is carried out and the benefits of reducing carbon emissions are discussed in relation to a tax on emissions. 3.0 Greenhouse Model 3.1 Interior Microclimate The software program TRNSYS, a Transient System Simulation Program, was utilized to simulate the greenhouse microclimate. TRNSYS is a simulation environment for the transient simulation of energy systems and encompasses multi-zone buildings [18]. A 3-dimensional greenhouse rendering was created utilizing a Type 56 Multi-Zone Building component and the TRNSYS sub-program TRNBuild. A venlo-type greenhouse was modelled with a plan area of 0.4 hectares, gutter height of 5.5 metres (m), 10 bays each with a width of 5 m, and a roof slope of approximately 25. Double-polyethylene cover material, commonly used in Southern Ontario, was utilized with an overall heat transfer coefficient of 3.2 W/ m 2 K, which is within the range of values reported by others [19]. A series of lower and upper thermal zones were created within the greenhouse to simulate conditions within and above the crop, as shown in Figure 1. A constant crop density of 6 kg/ m 2 was considered and the thermal properties of the crop are assumed to be similar to those of water. 23

39 Figure 1 3-Dimensional Greenhouse Model [20] A schematic layout of the controls involved in the microclimate simulation is shown in Figure 2. Based on recommended growing conditions and discussions with greenhouse operators, the day and night indoor set point temperatures were set at 22 C and 18 C, respectively. In addition, heat is supplied between the hours of 3 and 6 am year-round, regardless of the indoor temperature, to facilitate morning pre-heating prior to sunrise. Infiltration was set at 0.5 air changes per hour (ACH) [21,22]. When the indoor temperature exceeded 25 C or relative humidity exceeded 85%, ventilation through the greenhouse cover vents was activated and set at a rate of 60 ACH [21]. Thermal curtains close when global solar radiation is less than 5 W /m 2 during the winter months. During the summer months the curtains are not utilized as outdoor ambient temperatures during the night are generally above 15 C and heat retention is not necessary. Indoor air velocity was assumed to be 0.15 m/s based on estimates by others [23,24]. In order to avoid instabilities in the simulation, coupling air flows between lower and upper thermal zones was set at 1 ACH. Similarly, coupling air flows between lower zones was set at 1 ACH at the beginning of the growing season and gradually decreased to simulate the stagnation of horizontal air movement as the crop density increases, as shown in Figure 3. Crop evapotranspiration plays an important role in the energy balance of the greenhouse microclimate [25,26] and was accounted for in the model via an internal humidity gain. It has been assumed that during the summer months when solar radiation is highest and the crop is nearing maximum height, 60% of solar radiation reaching the interior of the greenhouse is converted to latent heat by transpiration [23,27,28]. This value was set at 20 grams of water /hour /m 2 at night. A high-pressure fogger is used to maintain the indoor relative humidity between 75 and 85%. 24